castle: the heredity of sex. 205 



D. Exceptional Parthenogenesis in Bombtx mori, etc. 



Occasional parthenogenesis is known to occui' in certain Lepidoptera, 

 when the mother is forcibly prevented from copulating. The cases 

 which have been most carefully studied are those of the silk moth, 

 Bombyx mori, and the gypsy moth, Ocneria dispar. The unfertilized 

 as well as the fertilized eggs of these species are known, through the in- 

 vestigations of Platner ('88) and Henking ('92), to undergo tivo matu- 

 ration divisions. But only an occasional unfertilized egg develops to the 

 larval stage, — only one in several hundred, or even one in thousands. 

 A still smaller proportion attain the condition of imagos. These few, 

 however, are of both sexes, and are capable of reproduction when bred to 

 ordinai'y individuals (von Siebold, '56). 



But it is entirely possible that in the very exceptional eg^ which de- 

 velops normally, a second maturation division has for some reason failed 

 to take place, or after it has taken place, a reunion has occurred of the 

 second polar nucleus with the egg nucleus, as sometimes in the egg of 

 Artemia, according to Brauer. Such a reunion would bring together 

 again the sex-characters segregated in maturation, and would produce 

 the physiological and morphological equivalent of the cleavage nucleus 

 of a fertilized egg. A similar result would follow the complete sup- 

 pression of a second maturation division. 



The occurrence of individuals of both sexes among the partheno- 

 genetic offspring of the silk moth and gypsy moth shows that in these 

 species, as in other normally dioecious animals, there is no uniform 

 dominance of one sex over the other, such as we find occurrins: amone: 

 normally parthenogeuetic animals, where the female character regularly 

 dominates. 



V. Abnormal Sex Proportions among Hybrids. 



Bateson and Saunders (: 02, p. 139) consider it as "on the whole 

 against the hypothesis that sex depends chiefly on gametic differentiation 

 that the statistical distribution of sex among first crosses shows great 

 departure from the normal proportions." The writer does not share this 

 opinion, for on the hypothesis of sex advanced in this paper departures of 

 the sort indicated are capable of ready explanation. 



It should be stated, however, that the known cases of this sort ai-e 

 comparatively rare, whereas the statement of Bateson and Saunders 

 might lead one to expect their frequent occurrence. The writer knows 

 of but two cases about which our information is full enough to warrant 

 statistical examination. 



